Conventional sector servo direct access storage devices ("DASDs") have a fixed number of servo sectors (N) with the disks spinning at a fixed rotation speed (w revolutions per second). The sampling rate (or frequency) f.sub.S corresponds to N*w samples per second. The servo bursts in the servo sectors are referred to herein as a stream of positioning bursts, and the bursts of this stream are therefore sensed by a sensing device at the rate at which they move past the sensing device, or f.sub.S. Digital servo controller designs are usually implemented assuming this sampling rate.
Seek acoustic noise generated by a DASD mechanism is a significant problem. The acoustic noise generated during seek arises from broadband forcing of the environmental components or from the excitation of "pure tone" modes at a well defined frequency f.sub.S. It is critical to minimize seek acoustics without compromising the access performance of a DASD. Pure tone modes due to structural resonance can get amplified through the servo feedback process or due to roughness (significant step change) in the digital-to-analog converter ("DAC") output value.
The configuration of a typical DASD servo is shown in FIG. 1. An exemplary DASD 10 is depicted having a sensing device (actuator 14 coupled to read/write electronics 12). It should be understood to those skilled in the art that the terms "sensing device" and "access device" used herein should be construed broadly and without limitation to connote parts of the same subsystem within the DASD, or components of different subsystems. An access device (actuator 14, voice coil motor ("VCM") 15, and spindle/VCM Driver 22) is also depicted.
A position error signal ("PES") 32 (derived from a sensed position burst via line 30) from peak detecting demodulator 16 is initially sampled by an analog-to-digital converter 26 within a microprocessor unit ("MPU") 18 and sent to the hard disk controller ("HDC") 20 for PES non-linearity processing 24, where known distortion of the PES is first corrected (by the HDC without burdening the MPU). The linearized PES is sent back to the MPU for conventional seek servo computation in Seek/Settle/Tk-Follow subsystem 28. During seek, a velocity servo technique is used to produce an output positioning control value (referred to herein simply as a positioning value) denoted by U.sub.n where n is the sampling instant. For every sector denoted by n-1, n, n+1 . . . etc., a new control output or positioning value is computed (possibly provided by a DAC, and typically with some time delay) and sent to the access device via line 34. The positioning values 34 are therefore calculated as a function of the sensed positioning (servo) bursts stored on the storage medium.
What is required is a method and system which reduce the noise associated with the fixed frequency sampling rates of conventional servo systems, such as the one depicted in FIG. 1. The servo computation complexity should be minimized to maintain the use of low cost MPUs in DASD designs.